The installation of offshore platforms is relevant in connection with the development of oil and gas fields at sea.
The removal of offshore platforms, consisting of deck and jacket, is a field which will become more important in the coming years, with the decommissioning of a number of offshore platforms.
Offshore platforms can be removed by being split up into smaller parts, for example by means of cutting torches or blasting, whereupon each of these smaller parts can be lifted aboard a barge or a ship and transported away. Regardless of how the splitting up is carried out, however, the costs associated with the breaking up and subsequent transport in smaller parts are substantial.
Thus, it is a wish to split up a platform which has to be removed into the fewest possible parts, in which case a natural division is to divide it into deck and jacket, each of which is removed separately. In the case of small platforms with low weight this is relatively problem-free, while in the case of large and heavy platforms it entails lifting with extremely large and expensive crane vessels, if it is even possible at all within the scope of what can be implemented in practice.
Norwegian patent application no. 91 0358 describes a method for removal of a platform jacket, wherein a device in the form of a ballastable hull with a deck which is arranged to receive the jacket is rotated into an approximately vertical position in which the deck is located parallel to the platform jacket. The platform jacket is secured to the deck and released from the seabed, whereupon the hull is rotated by means of ballasting to a position where the deck and the jacket arc located horizontally in the water. The device has a U-shaped cross section, and in addition to the deck comprises two long sides arranged at the side of the deck. With the method and the device according to Norwegian patent application no. 91 0358 it is thereby possible to remove a platform jacket from the seabed. The described method and device do not, however, solve the whole problem of removing a platform jacket, since during the transfer of the jacket from the device to a quay or a barge the same problem again arises as to how to move such a heavy structure without dividing it up into smaller parts.